Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, small pore silicoaluminophosphates (SAPOs), such as SAPO-34 and SAPO-18, have shown particular promise. SAPO-34 belongs to the family of molecular sieves having the framework type of the zeolitic mineral chabazite (CHA), whereas SAPO-18 belongs to the family of molecular sieves having the AEI framework type. In addition to regular ordered silicoaluminophosphate molecular sieves, disordered structures, such as planar intergrowths containing both AEI and CHA framework type materials, are known and have shown activity as oxygenate conversion catalysts.
It is also known that silicoaluminophosphates of relatively small particle size are particularly effective in the conversion of methanol to olefins. Thus, for example, De Chen, et al., reports that SAPO-34 crystals of 0.4 to 0.5 μm gave the largest capacity of olefin formation (Microporous and Mesoporous Materials, 29, 191-203, 1999). In this work, the crystals were obtained from a single batch of crystals, which was fractionated to obtain the differently sized crystals evaluated.
U.S. Pat. No. 5,126,308 discloses that ELAPO molecular sieves, wherein EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, composed of particles at least 50% of which have a particle size less than 1.0 μm and no more than 10% of which have a particle size greater than 2.0 μm, exhibit improved selectivity and catalyst life in the catalytic conversion of methanol to olefins.
International Patent Publication No. WO03/048084, published Jun. 12, 2003, discloses that increased selectivity to ethylene and propylene is obtained in the catalytic conversion of methanol to olefins when the catalyst comprises an ELAPO molecular sieve, wherein EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, having a platelet-type crystal morphology, wherein the average smallest crystal dimension of the crystals is at least 0.1 micron and the aspect ratio of the crystals is less than or equal to 5.
Thus, in order to synthesize silicoaluminophosphate molecular sieves having CHA and/or AEI framework types that are effective in the catalytic conversion of oxygenates to olefins, it is important to be able to control not only the framework structure of the molecular sieve but also its crystal size and in particular to be able to reliably produce materials having a small average crystal size and a low variation in the size between individual crystallites.
The synthesis of silicoaluminophosphate molecular sieves having CHA and/or AEI framework types involves mixing reactive sources of silicon, phosphorus and aluminum in the presence of water and an organic directing agent, particularly tetraethylammonium hydroxide (TEAOH), then heating the mixture to a crystallization temperature, typically 150° to 250° C., and thereafter maintaining the mixture at the crystallization temperature for up to 150 hours. Although a variety of suitable silicon, phosphorus and aluminum sources are available, most commercial sources contain significant levels of impurities, including metals. For example, certain commercial sources of TEAOH contain as much as 1.6 wt % potassium and 0.05 wt % of sodium, whereas phosphoric acid, a preferred phosphorus source, frequently contains in excess of 0.01 wt % of alkali metal.
Study of the synthesis of silicoaluminophosphate molecular sieves has shown that the level of alkali metal, particularly sodium and potassium, impurities in the synthesis mixture has an important role in determining the average crystal size and the crystal size distribution of CHA and AEI-containing materials. In particular, it has now been found that materials having an average (d50) crystal size of less than 2.2 micron and a relatively low crystal size distribution [(d90−d10)/d50=<1.0] can be obtained with synthesis mixtures containing between about 0.08 and about 0.9 g of alkali metals/mole of alumina. In contrast, when the alkali metal concentration increases above 0.9 g/mole of alumina, the crystal size variation increases whereas, when the alkali metal concentration falls below about 0.08 g/mole of alumina, there is a loss in crystal size reproducibility.
In addition, in the synthesis of AEI/CHA intergrowths, it is found that the level of alkali metal impurities in the synthesis mixture has an important role in determining the composition of the intergrowth. Thus, when the alkali metal concentration in the synthesis mixture is less than 1.0 g/mole of alumina, the production of CHA-rich materials is favored, whereas AEI-rich materials tend to be produced at alkali metal concentrations of at least 1.0 g/mole of alumina.
Synthesis of silicoaluminophosphate molecular sieves, including SAPO-34, is described in U.S. Pat. No. 4,440,871. According to this patent, in synthesizing these SAPO compositions, it is preferred that the reaction mixture be essentially free of alkali metal cations and have a composition expressed in terms of molar oxide ratios is as follows:aR2O:bM2O:(Six Aly Pz)O2:cH2Owherein “R” is an organic templating agent, “a” has a value great enough to constitute an effective concentration of “R” and is within the range of >0 to 3; “b” has a value of zero to 2.5; “c” has a value of from zero to 500, preferably 2 to 30; “x”, “y” and “z” represent the mole fractions, respectively of silicon, aluminum and phosphorus in the (Six Aly Pz)O2 constituent, and each has a value of at least 0.01. Apart from pH control, no specific reason for the preference for alkali-free reaction mixtures is given in U.S. Pat. No. 4,440,871.
U.S. Pat. No. 5,741,751 discloses a process for preparing silicoaluminophosphate molecular sieves, such as SAPO-11, from a reaction mixture containing an active source of phosphorus and a particulate hydrated alumina having an average particle size of less than about 40 micron, a particle density of less than about 1.0 g/cm3 and an alkali content of less than 0.12 wt %. There is no disclosure of producing CHA and/or AEI framework-type materials.
International Patent Publication No. WO03/048042, published Jun. 12, 2003, discloses a process for the manufacture of a SAPO-34 crystalline molecular sieve by hydrothermal treatment of a surfactant-free synthesis mixture containing a structure-directing agent and sources of silicon, aluminum and phosphorus, wherein the source of silicon is a tetraalkyl orthosilicate. The resultant SAPO-34 crystalline molecular sieve has a mean particle size of at most 400 nm.
International Patent Publication No. WO03/048043, published Jun. 12, 2003, discloses a process for manufacturing a silicoaluminophosphate crystalline molecular sieve, such as SAPO-34, having a mean particle size of at most 400 nm by crystallizing a synthesis mixture comprising sources of aluminum, phosphorus and silicon, wherein the source of silicon is in solution with a water-miscible organic base.
U.S. Pat. No. 7,375,050discloses a process for producing silicoaluminophosphate molecular sieves, such as CHA framework type materials, in which the average particle size is consistently 1 μm or less and the particle size distribution is such that up to 80% (by number) of the particles are within ±10% of the mean. The process involves crystallization of a synthesis mixture comprising a source of aluminum, a source of phosphorus, at least two organic templates R1 and R2, optionally a source of silicon, and seeds, wherein the molar ratio of organic template (R+R2) to aluminum (Al) in the synthesis mixture is ≦1.25.
U.S. Pat. No. 6,334,994 discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed phase composition. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown forms of AEI and CHA framework type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30. RUW-19 is synthesized by initially mixing an Al-source, particularly Al-isopropoxide, with water and a P-source, particularly phosphoric acid, and thereafter adding a Si-source, particularly colloidal silica and an organic template material, particularly tetraethylammonium hydroxide, to produce a precursor gel. The gel is then put into a steel autoclave and, after an aging period at room temperature, the autoclave is heated to a maximum temperature between 180° C. and 260° C., preferably at least 200° C., for at least 1 hour, with the autoclave being shaken, stirred or rotated during the entire process of aging and crystallization. The resultant RUW-19 crystals are said to have a crystal size between 0.001 and 10 microns.
International Patent Publication No. WO 02/70407, published Sep. 12, 2002, discloses a silicoaluminophosphate molecular sieve, now designated EMM-2, comprising at least one intergrown form of molecular sieves having AEI and CHA framework types, wherein said intergrown form has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the molecular sieve. Synthesis of the intergrown material is achieved by mixing reactive sources of silicon, phosphorus and a hydrated aluminum oxide in the presence of an organic directing agent, particularly a tetraethylammonium compound. The resultant mixture is stirred and heated to a crystallization temperature, preferably from 150° C. to 185° C., and then maintained at this temperature under stirring for between 2 and 150 hours. The resultant crystals are said to have plate-like morphology with an average smallest dimension of less than 0.1 micron and a ratio of the largest to smallest dimensions of 2 to 20.